The present invention relates to x-ray inspection using Computed Tomography (CT) and, more particularly, to a novel method and apparatus for inspecting large objects such as a gas turbine engine component, rocket engine component, or the like, using a rotate/rotate CT x-ray inspection system.
CT systems are known, and include a source of radiation and an associated detector both of which can be moved, relative to an object under inspection for purposes of reconstructing a cross-sectional area or slice through the object at a selected location on the object by means of penetrating the object with radiation and detecting the attenuation of the radiation caused by the object on an opposite side of the object from the radiation source. The resolution and quality of the image of the cross-sectional slice of the object is dependent upon the precision with which the relative positions of the radiation source, detector and object remain constant during examination of the object. Therefore, most CT systems, such as those used in medicine, have the radiation source, detector and object fixed in position relative to one another while the attenuated radiation signals are detected to reconstruct the cross-sectional slice of the object. Maintaining the relative positions of the radiation source, detector and object is not possible, however, when the object under test is so large that it cannot completely fit within the field of view or fan angle of an x-ray source/detector system.
Presently known x-ray inspection systems for large objects, such as those systems disclosed in U.S. Pat. Nos. 4,422,177; 4,600,998; and 3,766,387, typically inspect the component or part by a rotate/translate method to obtain complete data for reconstruction of an image of a selected cross-sectional area of the component. Referring to FIG. 1, the rotate/translate method involves positioning the component at an initial angle of orientation relative to an x-ray beam 12 and then translating the component 10 linearly through the x-ray beam 12 along a line of travel, indicated by broken line 14 in FIG. 1, substantially perpendicular to a centerline 16 between the focal point 18 of the x-ray source 20 and the center of a detector 22. Attenuated x-ray beam data is collected by the detector as the component is translated or passed linearly through the beam 12. The component is then incrementally rotated about a component inspection rotational axis 22 to a new angular orientation relative to x-ray beam 12; then the component is translated again through x-ray beam 12 while data is collected on the attenuation of the beam by detector 20. This process is repeated until sufficient data is accumulated to construct an image of the cross-sectional slice through the component.
Typically, before each translation through the x-ray beam, the component is rotated through an angle alpha about the same as the fan angle of the x-ray system, or less, to provide sufficient data to reconstruct the cross-sectional image. This process can require that the component be incrementally rotated and translated several times to obtain sufficient angular data to generate an image of sufficient resolution and quality. If the component is rotated by about the x-ray beam fan angle before each translation through the x-ray beam, the minimum number of passes through the x-ray beam required to obtain sufficient data to reconstruct a cross-sectional image will be equivalent to the sum of the fan angle plus 180.degree. divided by the fan angle and rounded to the next highest integer. For example, if the fan angle is about 20.degree., then at least 10 passes through the x-ray beam would be required to reconstruct the cross-sectional image.
Thus, the rotate/translate method can require considerable mechanical manipulation to acquire sufficient data to reconstruct a cross-sectional image with good resolution. Data acquisition can therefore be quite time consuming, depending upon the size of the part, the fan angle and the angle by which the part is incrementally rotated before each pass through the x-ray beam. In high volume production processes, such as gas turbine engine manufacturing or the like, inspection of very large components by the rotate/translate method can be very inefficient.
Another disadvantage of using the rotate/translate method for inspection of large objects is that the component geometry does not remain at a constant distance from the x-ray focal point as the component is translated linearly through the x-ray beam. This requires additional computer software and computational time to adjust the data to compensate for the change in component geometry before the cross-sectional image is reconstructed.